The present disclosure generally relates to complexation of metal ions and, more specifically, to dissolution of minerals, chelating agents suitable for use in conjunction with various subterranean treatment operations, and methods for synthesizing such chelating agents.
Treatment fluids can be used in a variety of subterranean treatment operations. Such treatment operations can include, without limitation, drilling operations, stimulation operations, production operations, remediation operations, sand control treatments, and the like. As used herein, the terms “treat,” “treatment,” “treating,” and grammatical equivalents thereof refer to any subterranean operation that uses a fluid in conjunction with achieving a desired function and/or for a desired purpose. Use of these terms does not imply any particular action by the treatment fluid or a component thereof, unless otherwise specified herein. More specific examples of illustrative treatment operations can include, for example, drilling operations, fracturing operations, gravel packing operations, acidizing operations, scale dissolution and removal operations, sand control operations, consolidation operations, and the like.
Acidic treatment fluids are frequently utilized in the course of conducting various subterranean treatment operations. Illustrative uses of acidic treatment fluids during subterranean treatment operations include, for example, matrix acidizing of siliceous and/or non-siliceous formations, scale dissolution and removal processes, gel breaking, acid fracturing, and the like.
Acidizing operations may be performed to stimulate a subterranean formation and increase production of a hydrocarbon resource therefrom. During an acidizing operation, an acid-reactive material in the subterranean formation can be at least partially dissolved by one or more acids to expand existing flow pathways in the subterranean formation, to create new flow pathways in the subterranean formation, and/or to remove acid-soluble scale in the subterranean formation. Acidizing a subterranean formation's matrix can be particularly effective for stimulating production.
The material being reacted with an acidizing fluid can significantly dictate how an acidizing operation is performed. When acidizing a non-siliceous substance, such as a carbonate material, mineral acids, such as hydrochloric acid, may often be sufficient to affect dissolution. Organic acids may be used in a similar manner to hydrochloric acid when dissolving a non-siliceous substance, especially at temperatures exceeding about 180° C. Siliceous materials, in contrast, are only readily dissolvable using hydrofluoric acid, optionally in combination with other acids to maintain a low pH environment. As used herein, the term “siliceous” refers to a substance having the characteristics of silica, including silicates and/or aluminosilicates. Illustrative siliceous materials can include, for example, silica, silicates, aluminosilicates, and any combination thereof, optionally in further combination with a non-siliceous substance, such as a carbonate material. Most sandstone formations, for example, contain about 40% to about 98% sand quartz particles (i.e., silica), bonded together by various amounts of cementing materials, which may be siliceous in nature (e.g., aluminosilicates or other silicates) or non-siliceous in nature (e.g., carbonates, such as calcite).
Carbonate formations contain minerals that comprise a carbonate anion. Calcite (calcium carbonate), dolomite (calcium magnesium carbonate), and siderite (iron carbonate) represent illustrative examples. When acidizing a carbonate formation, acidity of the treatment fluid alone can often be sufficient to solubilize the carbonate material by converting the carbonate anion into carbon dioxide and leaching a metal ion into the treatment fluid. As the concentration of dissolved metal ions rises, particularly at higher pH values upon spending of the acid, the solubility limit of the metal ions may be exceeded and precipitation of scale may occur.
Siliceous formations can include minerals such as, for example, zeolites, clays and feldspars. As indicated above, siliceous formations are usually acidized with hydrofluoric acid, optionally in combination with another acid, in order to react the siliceous minerals and affect their dissolution. Dissolved silicon species can be particularly prone toward undergoing secondary reactions with alkali metal ions to form highly damaging alkali metal silicate precipitates. Co-present non-siliceous minerals, such as carbonate minerals, may be concurrently dissolved while acidizing a siliceous material and lead to further precipitation issues.
Calcium ions and other alkaline earth metal ions can be particularly problematic when acidizing either siliceous or non-siliceous formations. As indicated above, the solubility limit of the metal ions may be quickly exceeded and deposition of scale may occur upon spending of an acid. In the case of siliceous formations acidized with hydrofluoric acid, calcium ions liberated from a co-present carbonate material can react readily with free fluoride ions to form highly insoluble calcium fluoride. Other metal ions can prove similarly problematic in this regard, either by forming an insoluble reaction product with hydrofluoric acid or by themselves upon forming an insoluble material under the particular conditions present in a wellbore. Calcium fluoride and other types of scale formed from metal ions can be highly damaging to subterranean formations, possibly even more so than if the initial acidizing operation had not been performed in the first place.
One approach that has been used to address the issues associated with dissolved metal ions is to employ chelating agents, which can sequester the metal ions in a more soluble and less reactive form of a metal-ligand complex. As used herein, the terms “complex,” “complexing,” “complexation” and other variants thereof refer to the formation of a metal-ligand bond without reference to the mode of bonding. Although complexation of metal ions may involve a chelation process, complexation is not deemed to be limited in this manner. Once bound in a metal-ligand complex, the metal ions may have a significantly decreased propensity to undergo a further reaction to form scale.
There are difficulties associated with chelation strategies, however. At low pH values, the carboxylic acid groups of many chelating agents may be substantially protonated, a form that can be ineffective for promoting metal ion complexation. This issue can significantly limit the working pH range over which an acidizing operation may take place, potentially limiting the acidizing operation's speed and effectiveness (i.e., by working at higher pH values). In addition, many chelating agents are commercially supplied in their alkali metal salt forms, which can be especially problematic for siliceous formations due to the precipitation issues noted above. Conversion of the alkali metal salt form of a chelating agent into the free acid form or an alternative salt form can often be problematic and/or expensive, particularly at the large scales needed to support subterranean treatment operations. Although numerous chelating agents are known, there are presently a very limited number available in suitable form and in sufficient supply to support widespread downhole use. Provisions for working with the less desirable alkali metal salt forms of commodity chelating agents presently may need to be made in order to facilitate their use in subterranean treatment operations.